I understand that caps store electricity and they can pass AC but not DC.

My question is why are capacitors used as couplers to the grid . Would the capacitor not block the circuit and act as an off switch, after all they do not pass DC?

I am sure I am missing something.

M Khan

Hi M Khan

Signals between stages vary in amplitude, like AC. Capacitors will pass this AC signal while blocking DC plate voltage on one stage from getting to grid of the next.

Signal is AC, and power/bias is DC. The coupling capacitor strips off the DC (blocks it) while allowing the signal to pass. Bypass capacitors are often used in the cathode (or emitter) circuits of audio amplifiers where they bypass the bias resistor.

Well...

If you imagine a tube with a rubber diaphram in the middle of it, full of water. If you push more water into one end, the diaphram distorts with the pressure. Because it has distorted now, it pushes a little water on the other side of it out of the tube.

If you relax the pressure, the diaphram returns to normal and so you now have a negative pressure on the other side - it sucks a little water back into the tube.

Apply this to a capacitor and electricity - as the AC voltage pressure rises to a peak, the diaphram expands and pushes electrons out the other side - you apparently have a flow of current. As the AC pressure drops to zero, electrons are pulled back in to fill the now negative pressure. As the AC cycle goes below zero and peaks at a negative value, so the diaphram distorts the other way and pulls more electrons in. The AC cycle then returns to zero and the whole process repeats

If you just push a DC current in, the diaphram distorts, making a positive pressure on the other side - but it can only go so far, so when the diaphram is at maximum, you could see it as charged and since no more distortion of the diaphram is possible (without damage), current flow stops also.

Now if we added a dye into the water, say red on one side, and blue on the other, because the diaphram is in the middle, the dyes won't mix (see it as an isolator) - there is no direct connection between the left and right sides of the diaphram and so the two dyes are still isolated. It can be seen that it conducts only a change in pressure.

Even though you have no direct flow, you can see how an AC current is conducted now, yes?

Can you see how it would work with a pulsed (or constantly varying) DC current too?

This analogy also works for the term "leaky capacitor" - if you had a tiny hole in the diaphram, there WOULD be a small current allowed to flow. There would be a flow of current through the hole proportional to the size of the hole. If it was very small, the resistance through the hole would be quite high, but the AC operation of the diaphram wouldn't be affected too much, so although it would still very much act as a capacitor, there would be a resistance also. All due to a leaky diaphram

Of course, in a real capacitor, the plates don't really distort and the diaphram is the insulator (or "dielectric") I just use this an an analogy.

Hope it helps!

Oh, yes they do! Distortion is what makes them (some of them, anyway) microphonic, and the converse can also happen - they can generate vibrations/sound as they are charged or discharged.

Brett

well yes, but then it gets confusing lol

The thing you need to understand is that active electronic devices like tubes and transistors require their terminals to be at definite baseline DC voltages (or currents) in order for them to function as intended. Setting up these prerequisite DC conditions is called biasing. It is usually accomplished by adding one or more resistors to the circuit so voltage drops occur in such a way as to create the desired conditions.

When coupling circuits together, one has to make sure that the bias voltages in one stage don't throw off the bias voltages in the next stage. This is readily apparent in vacuum tube electronics where plate voltages are positive a good amount with respect to cathodes, while grid voltages are usually a couple of volts negative with respect to cathodes. By their very nature, capacitors pass AC but not DC, so they're ideal coupling components. The signal that we want to carry from one stage to the next is AC, so it will pass through a coupling capacitor with ease. But the DC bias voltages needed to keep the tubes or transistors 'happy' stay where they belong in each stage.

Transformers also pass AC but block DC, so they can be used for coupling stages together as well. Indeed, they were used quite extensively in 1920s radios because in addition to coupling, they can also provide a voltage step-up or "amplification" which does not occur in capacitor coupling circuits. However, by the late 1920s, tubes having more amplification were developed, and manufacturers were beginning to look for places to cut corners, so coupling transformers fell out of favor except for certain circumstances where a DC path had to be maintained, or where impedance matching was crucial.

Thanks all for the excellent explanation. It cleared my fundamentals.

M Khan

Simply speaking (since I am a physicist and the question is about the physics )

The reactives behave this way:

1. Capacitors resist voltage change. (and store electric charge). Mechanical analogy - a coil spring (stores static energy of compression/expansion).
2. Inductors resist current change. (and store magnetic field). Mechanical analogy - mass, inertia (stores the kinetic energy).

This said, the capacitor, resisting the voltage change, will try to keep the constant voltage between the two plates. Meaning any dV change of a voltage on the "left" of the cap (imagine a typical schematic with a cap use to DC decouple a grid) will be passed to the right; hence you AC signal getting through.
However capacitor does not pass through DC (no voltage change), so the DC levels stay different on either side of a cap. Hence your DC bias on one side does not affect the DC bias on the other side.

A good picture to keep in mind (a Russian joke about the student explaining the AC current going through a cap and DC not going to the professor):
DC current, a straight line, cannot cross a divide, but AC current, being a curved sine wave, goes around the cap, like a snake.

Cheers!

Those are all great answers. Cleared the fog nicely. Thanks, lyle.

Dragon:

The diaphragm analogy is excellent...

Yes, thank you Dragon. I've often used water as an analogy for electricity, but I'd never heard a useful "water model" for a capacitor!